1. Field of the Invention
The present invention relates to surface-coated cutting inserts which can be attached to various types of insert-removable machine tools and used for cutting. The present invention additionally relates to a method for manufacturing such inserts.
The present invention claims priority from Japanese Patent Application No. 2005-349652, filed on Dec. 2, 2005, the content of which is incorporated herein by reference.
2. Background Art
Cutting inserts have an insert body of, for example, a polygonal shape composed of a hard material substrate such as tungsten carbide-based cemented carbide, titanium carbonitride-based cermet or ceramics, with cutting edges formed in an intersecting edge line region where rake faces and flank faces intersect. Such cutting inserts are attached to insert-removable machine tools and widely used for cutting metallic materials. In addition, surface-coated cutting inserts have also been proposed to date, primarily to enhance wear resistance. Such cutting inserts are obtained by coating a cutting insert with, in order from the insert body side, a base layer formed of a single layer or a plurality of two or more layers composed of a carbide, nitride, oxide or boride of a periodic table Group IVa metal, Group Va metal, Group VIa metal, aluminum or silicon, or a complex compound thereof, on the surface of the insert body; an intermediate layer including primarily Al2O3; and an outermost layer which, as with the base layer, is formed of a single layer or a plurality of two or more layers composed of a carbide, nitride, oxide or boride of a periodic table Group IVa metal, Group Va metal, Group VIa metal, aluminum or silicon, or a complex compound thereof.
Patent Documents 1 and 2, for example, describe such surface-coated cutting inserts in which portions of these layers have been mechanically removed. Patent Document 1 specifically describes a cutting insert having an α-Al2O3 layer (intermediate layer) from which a TiCxNyOz layer (outermost layer) is mechanically removed only from the cutting edge lines or from both the rake faces and the cutting edge lines, but is left in place on the flank faces. By thus having Al2O3, with its strong resistance to diffusion-type crater wear and smearing, serve as the outermost layer on the rake faces and the cutting edges, and by having TiCxNyOz, with its strong resistance to flank wear, serve as the outermost layer on the flank faces instead of Al2O3 which wears relatively fast on the flank face, an excellent wear resistance is achieved at the same time on both the rake faces and the flank faces. This prior art also discloses that the TiCxNyOz layer on the flank faces facilitates “used cutting edge identification” for distinguishing between used cutting edges and unused cutting edges. Illustrative methods for mechanical layer removal include brushing, polishing and blasting.
Patent Document 2 provides a wear-indicating layer on the flank faces for easy detection of whether a cutting edge is used or unused. Also disclosed is a more preferred method in which an Al2O3 layer is applied as a wear-reducing coating and a TiN layer is subsequently applied as an outermost layer, after which the TiN layer, which is intended to serve as a wear-indicating layer, is mechanically removed by brushing or sandblasting from the rake faces.
However, in such cutting technology, there has been a desire in recent years for ever higher precision machining. To this end, in addition to the concern that has obviously existed to date regarding the dimensional precision of machining, much importance is now being placed on enhancing the quality of the machined surface (finished surface) of the workpiece.
Here, as noted above, because prior-art cutting inserts have been provided with an excellent wear resistance at both the rake faces and the flank faces, a certain degree of improvement has been achieved with regard to cutting precision.
However, because the criteria for judging the quality of the machined surface have become more exacting in recent years, at the initial stage of cutting or at a stage relatively soon after the start of cutting, the cutting edge quickly reaches a condition where the desired quality cannot be achieved. Even though little wear has yet occurred, the tool operator decides at this point that the cutting edge has reached the end of its useful life, and either changes the cutting edge in use or changes the insert itself. Here, the operator determines that the cutting edge has reached the end of its useful life when the machined surface of the workpiece incurs scuffing and fluffing, becoming perceptibly rough to the touch, or the surface appears dull and without luster. In particular, with the trend recently toward more difficult-to-machine materials, such abnormal surface conditions are increasingly likely to occur.
Also, in the above-described prior-art cutting inserts, although “used cutting edge identification” for distinguishing between used cutting edges and unused cutting edges is carried out based on the TiCxNyOz layer or the TiN layer serving as a wear-indicating layer on the flank face sides, it is difficult to reliably determine whether a cutting edge is used or unused under, for example, poor lighting conditions in the operational sites of cutting work. In particular, the above-described surface-coated cutting inserts of Patent Document 2 rely on the fact that, due to discoloration under the action of heat generated during cutting, the flank face zone which does not come into direct contact with the workpiece during use but is adjacent to the cutting edge that has been used also discolors. Accordingly, the region that discolors is smaller than the rake face which comes into direct contact with the chip and generates much heat, and the degree of discoloration is also weak, making it even more difficult to identify whether a cutting edge has been used or is unused.
It is therefore an object of the present invention to provide surface-coated cutting inserts which are capable of forming high-quality machined surfaces that have a good luster and do not cause scuffing or fluffing on the surface of the work material, and which enable a reliable determination to be made as to whether a cutting edge has been used or is unused. Another object of the invention is to provide a method for manufacturing such cutting inserts.    (Patent Document 1) Japanese Patent Application, First Publication No. H8-52603    (Patent Document 2) Japanese Patent Application, First Publication No. 2002-144108